Implement architecture-independent fallback in repositories.bzl

Archictecture-dependent binaries such as `protoc` limits portability to different targets (namely, I'm interested in Tensorflow ppc64le builds). I tried replacing x64 binaries with ppc64 in the cached dirs and managed to compile Tensorflow (see https://github.com/tensorflow/tensorflow/issues/10306).

I could gladly send a pull request that does so, but I need some help to get a better understanding of `bazel` first, if that's possible.

Start by reading repositories.bzl and the issue's description of architecture-dependent cached binaries, especially protoc and the desired ppc64le fallback. Trace how repository binaries are selected, then verify that the implementation permits architecture-independent fallback without replacing the target-specific behavior.

Written by the indexing model from the issue text.